rts/PosixSource.h: Define __USE_MINGW_ANSI_STDIO on Windows

This was removed in 8dc72f3c which cleaned up
PosixSource.h. Strangely, this only started breaking for me now.

Test Plan: Validate on Windows

Reviewers: simonmar, erikd, austin, Phyx

Reviewed By: Phyx

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2812

GHC Trac Issues: #12951